hw_features is what is possible; feat is what is turned on *or* other Plan
9 characteristics. Notably, NETF_PADMIN is more of a signal that the NIC
will pad to the mintu. It's not something the hardware will turn on or
off.

By #defining hw_features to feat, we were possibly turning on things that
shouldn't be on by default. Likewise, an = (instead of |=) could clobber
values of feat. It was all quite nasty.

Note that the name 'feat' is an ABI to some extent. etherbind() looks for
that string when it decides which features get added to the Ipifc. If we
have dynamic features and want them to actually take affect to IP stacks
bound to a NIC, then we'll need to propagate those changes to Ipifc.